In recent years, along with the development of technology for digitally recording moving pictures, still pictures, and other video data, large volumes of data are now being handled. CD or DVD and other optical disk devices are now in the limelight as large volume recording devices. Research for further increasing capacity is also underway.
FIG. 1 is a sectional view of a conventional optical disk.
Trenches 15a are provided in one surface of a light transmission type disk substrate 15 having a thickness of about 1.2 mm. An optical recording layer 16 comprised of for example a dielectric film, a recording film, another dielectric film, a reflection film, etc. stacked in that order is formed on this surface. The film configuration and the number of layers are different according to the type of the recording material and the design.
A protection layer 17 is formed above the optical recording layer 16.
When recording on or reproducing from the optical disk, a laser beam of a predetermined wavelength or other light is focused on the optical recording layer 16 from the disk substrate 15 side.
When reproducing from the optical disk, return light reflected at the optical recording layer 16 is received by a light receiving element, a predetermined signal is generated by a signal processing circuit, and a reproduction signal is extracted.
In the optical disk as described above, the optical recording layer 16 also has relief shapes corresponding to the trenches 15a provided in the surface of the disk substrate 15. These trenches 15a divide the track regions.
Below, regions projecting outward to the protection layer 17 side when seen from the disk substrate 15 will be referred to as “lands L”, while recessed regions will be referred to as “grooves G”.
In accordance with the design of the optical disk, one or both of the above divided lands L and grooves G are selected as the recording regions.
FIG. 1A is a sectional view of an optical disk of a format wherein for example only the land L regions are used as the recording regions, and the groove G regions are provided as regions for dividing the track regions.
Further, in other optical disks, there is also a format wherein only the groove G regions are used as the recording regions, and the land L regions are provided as regions for dividing the track regions.
Further, in order to improve the surface recording density, a land-groove recording format using both the regions of the lands L and grooves G as recording regions etc. has been developed.
This deals with the servo signal attenuation due to the optical MTF (modulation transfer function) hindering higher density in the track direction by using both of the lands L and the grooves G so as to obtain double the recording tracks of the cycle of generation of the tracking error signal, that is, the group pitch, and thus increase the recording density.
Further, a depth modulation recording format alternately arranges grooves having different depths to double the length of the cycle of generation of the tracking error signal so as to extend the optical limit and thus increase the recording density.
On the other hand, it is possible to make the inner surface forming the trench 15a formed in the disk substrate 15 meander, that is, form a so-called “wobble”.
For example, in one format of a MD (Minidisc), as shown in FIG. 1B, only the land L regions are used as the recording regions, the groove G regions are provided as regions for dividing the track regions, and grooves G formed with wobbles W and grooves G not formed with wobbles W and having inner surfaces extending linearly are alternately arranged.
From the above wobble W, a predetermined wobble matrix circuit or the like detects address information of the tracks, a clock for controlling the rotational speed of the disk, and the like. When recording address information or the like in the wobble, address information becomes unnecessary at the optical recording layer in the lands and grooves, so the recording density can be improved by that amount.
It is also possible to apply the wobble to the land-groove recording format and depth modulation recording format.
However, in order to prevent interference of adjacent track information due to optical MTF, it can be considered to form the wobble on one side of the wall surfaces of land-groove interfaces in the land-groove recording format or to arrange grooves having wobbles at a predetermined cycle similar to the cycle of the depth of grooves in the depth modulation recording format, but there is nothing prescribing the arrangement of wobbles. No optical disks having wobbles arranged desirably for the production process such as the mastering step, molding step, and a film formation step has yet been known.